The present invention relates to a mixer circuit used for frequency conversion of carrier frequency in radio communication system.
A mixer circuit is used for frequency conversion in a radio transceiver. A mixer circuit operates to multiply two analog signals for frequency conversion. A Gilbert cell type mixer circuit has been known for an integrated mixer circuit.
FIG. 8 shows a prior Gilbert cell type mixer circuit which used MOSFET elements. The symbols MN1 and MN2 show N-channel MOS transistors constituting a first differential pair 1. A gate of those transistors accepts local frequency signals LO (LO+, LO-) in differential form. The symbols MN3 and MN4 are N-channel MOS transistors constituting a second pair 2, having a gate similarly accepting local frequency signals LO (LO+, LO-). The symbols MN5 and MN6 are N-channel MOS transistors constituting a third pair 3A. Each gate of those transistors accepts high frequency signal RF (RF+ and RF-) in differential form, respectively. The symbol MN7 is an N-channel MOS transistor which operates as a current source. The symbols RL1 and RL2 are a load resistor common to the first pair 1 and the second pair 2.
In the Gilbert cell type mixer circuit in FIG. 8, the transistors MN5 and MN6 converts high frequency signal RF into differential current form, and each of the transistors MN1 through MN4 switches said current according to the local frequency signal LO, so that the product of the high frequency signal RF and the local frequency signal LO is obtained at one terminal of the loads RL1 and RL2 to provide intermediate frequency signal IF (IF+, IF-) in differential form.
By the way, a wireless transceiver operates with a battery, and it is preferable that voltage of the battery is as low as possible in view of small size and light weight of a transceiver.
However, the prior Gilbert cell type mixer circuit must have power supply voltage VDD at least 1.5-2.0 V, since the prior Gilbert cell type mixer circuit must have three stacks of transistors (MN1, MN5 and MN7), or (MN3, MN6 and MN7) et al.
If we try to decrease power supply voltage, high frequency operation is difficult since drain junction capacitance of a transistor increases, therefore, no application to wireless communication is possible.
As mentioned above, as the operation with low power supply voltage less than 1.5 V is impossible, a prior transceiver must have more than two series connected batteries each of which may be a primary battery (1.5 V) or a NiCd type secondary battery (1.2 V). Thus, small sized and/or light weight transceiver has been difficult because of the use of two series connected batteries.